Radio mirror



N. M. RUST ETAL Nov. 4, 1952 RADIO MIRROR sheets-sheet 1 Original FiledSept. 20, 19 t? I 45m; Known Nov. 4, 1952 RUST ETAL 2,617,030

RADIO MIRROR original Filed Sept 1947 3 SheeIs-Sheet 2 x a k x I I I I II I| I I l I I I I I I a m I I W\ Y\ W\\ \l I,

mvsm'ons NOEL M. RUST R MICHA L c. GREGORY ATTO RN EY Original FiledSept. 20, 1947 3 Sheets-Sheet 5 Nov. 4, 1952 MRUST AL 2,617,030

RADIO MIRROR INVENTORS NOEL M. RUST MICHAEL C. GREGORY ATTORN EYPatented Nov. 4, 1952 chael Craven Gregory,

Abercorn, Northern Rhodesia, assignors, by mesne assignments, to RadioCorporation of America, New York, N. .Y., a corporation of DelawareContinuation of application Serial No. 775,277, September 20, 19.47.This application August 14, 1951, Serial No. 241,816. In Great BritainMarch 21,1946

Section 1, Public Law 690, August 8, 1946 Patent expires March 21 1966up. 25o s3.63)

11 Claims. 1

This invention relates to radio mirrors, that is to say to mirror-likedevicesfor radiating very high frequency radio energy into space orreceiving such energy from space. Radio mirrors have numerousapplications among which are'ineluded radar.

This is a continuation of United States application Serial No. 775,277,filed September 2 1947, entitled Radio Mirrors, now abandoned.

For brevity of description the invention will be herein described withreference to the-transmission of radio energy. Since, however, aradiomirror is in essence a reversible device it will-be readily appreciated.by those skilledin the .art that the various constructions to bedescribed herein are-utilizable, ifdesired, for receptionas well as fortransmission.

The invention is primarily, though-not exclusively, concernedwith theuse of. fiat ,mirrorsfor giving, in conjunction-witha primary source ofradio energy,.a sharply directive beam. A typical and important, though.not by any means the only, case to which the invention maybeapplied isthat in which a primary source projects a beam of radiation on toaumirror which reflects it asa sharply directive beam. In this case, forhigh efficiency the whole beam is required tobe intercepted by themirror and this case will now be considered for the purpose ofexplaining the invention.

The invention is illustratedinand further explained in connection withthe accompanying drawings in which similar references are used forsimilarparts throughout.

Fig. l is a diagram used to explain certain optical laws pertinent to anunderstanding ofthe invention;

Fig. 2 is a diagrammatic side elevation view of one embodiment of theinventionhaving horizontal phase velocity varying plates;

Fig. 3 is a simplified elevationalview of a mirror or reflector employedinthe embodiment of Fig. 2;

Fig. 4. is a diagrammatic sectional view taken along the line YY of Fig.3;

Fig. 5 is a diagrammatic sectional .viewalong the1ineXX of Fig. 3;

Figs. 6 and '7 are respectively diagrammatic elevational and plan viewsof another embodiment of the invention employing horizontal steppedphase velocity varying plates;

Fig. 8 is a diagrammatic sectional view along the line Z- Z of Fig. 6;

Fig. 9 is a perspective view of still another embodiment of theinvention employing vertical phase velocity varying plates; and

Fig. 10 is a perspective view of a further embodiment of the inventionemploying stepped vertical phase velocity varying plates.

Referring firstto Fig. l which is an explanatory diagram it is well.known from a consideration of ordinary optical laws, which of courseapply both to light andto electromagnetic 'waves,;that the reflectedradiation from a plane mirror due to incident light thereon from a pointsource P in front .of the mirror may be regarded .as equivalent .todirect radiation .from,a virtual source V located behind the mirror, theactual and virtual sources being equidistant on opposite sides of themirror plane, and the reflected rays behaving as though they originated,without reflection, from the virtual source V. Now the wave front of theequivalent beam of radiation intercepted by the mirror is spherical'andthe present invention when applied to this case serves in effect toflatten this spherical wavefront at a particular plane relative to theactual mirror and which may be regarded as the equivalent aperture ofthe system.

Consider the spherical wave front at the remotest point B. (the upperedge in Figure 1) of the mirror M from the virtual s'ourceV. front whichis represented in broken line at- F is the segment of asphere drawnthrough the said point having the virtual source as centre and boundedby the extreme rays of the-beam touching the edges of the mirror. Theequivalent aperture. may be any plane such as AL'A2 or A3 (shown dotted)tangential to this spherical segment F and bounded by the intersectionwith the beam from the virtual source. .That'equivalent aperture Al (thenormal equivalent aperture) which is normal to-the central axis of thebeam from the virtual source V involves the least amount of correction(applied bythis invention) to achieve and therefore it is preferred toapply this inventionto achieve what may be termed a normal equivalentaperture The invention may, however, be applied to achievean equivalentaperture in any plane which fulfills the conditions herein outlined. Inthe case of the normal equivalent aperture the direction of maximumradiation will be that of the ray along the central axis. In the mostusual case, that in 'Which'the inventionis employed to provide anequi-ph'ase surface over the equivalent aperture, the direction of thebeam will be normal to the aperture plane. As will be seen later theinvention achieves its ends by differently modifying the velocity ofpropagation of the radio energy along dilferent rays from the actualsource either before the mirror is reached or after reflection from themirror or, preferably, both.

In the foregoing description the energy source has been assumed to be apoint source so as to simplify explanation. In practice, of course,point sources do not exist and, moreover, in many cases there is noattempt to approximate to a point source, for example the source mayapproximate to a line source. Nevertheless, the foregoing generalreasoning applies though its application in any particular case may bemore complex geometrically. In general the practical requirement is todirect as much as possible of a radiated beam of energy from the sourceto the mirror and then to reflect the beam in the required direction andwith the required degree of sharpness of directivity with as littleobstruction as possible by the original source. The general design ofsource and mirror is chosen, in each case, in dependence uponrequirements as to directivity but in arrangements as employed hithertothere has been the defect, more or less serious as respects attainmentof desired directivity, that the diiferent path lenghs from source toequivalent aperture introduce undesired phase change inequalities in theway pointed out above. The object of this invention is to avoid thisdefect and to provide mirror arrangements wherein a desired phaserelationship (usually, but not necessarily, equal) is obtained acrossthe equivalent aperture.

The invention is based on the fact that the phase of a field in a spacebounded on at least two sides by conductive surfaces placed parallel tothe electric field and not less than half a wave length apart progressat a velocity greater than the velocity in free space, this increase invelocity resulting from the inter-action of waves reflected from theboundary walls. This field velocity or phase velocity may be controlledby controlling the spacing of the boundary walls, the spacing being, ofcourse, always not less than a half wave length,

The present invention is based upon the abovementioned physicalphenomenon and consists in utilizing the said phenomenon to compensatefor the undesired effects of different path lengths from source toequivalent aperture so as to ensure that despite such difierent lengthsa desired phase condition-generally uniformityis obtained over theaperture.

According to this invention a radio mirror arrangement is provided withmeans for difierently modifying the velocity of propagation overdifferent paths from a wave guide end, aerial or other device associatedwith said mirror, to the equivalent aperture of said mirror (or viceversa) so that despite the different lengths of the said paths on thephase distribution across said aperture relative to the phase at thewave guide end or other device, a desired predetermined phasedistribution is obtained across said aperture.

Figure 2 is a diagrammatic side elevation of one embodiment of theinvention as applied to a horizontally polarized system requiring betterdirectivity in the horizontal plane than in the vertical plane; Figure 3is a simplified full view of the mirror employed in the embodiment;Figure 4 is a section on the line Y--Y of Figure 3 and Figure 5 is asection on the line XX of Figure 3.

Referring to Figures 2 to 5 the source of energy is a feed horn or waveguide flare P having a rectangular mouth which is of larger dimensionvertically than horizontally. (It may be noted that if the flare wererequired to be used for direct radiation into space without a mirror itwould, for the above-mentioned directivity requirements, be made withits larger dimension in the horizontal plane, 1. e. the plane in whichmaximum sharpness of beam is required.) The flare projects its beam onto a flat approximately elliptical mirror M positioned and inclined toreflect the beam back over the top of the flare so as just to clear itsupper (narrow) edge. In order to simplify Figure 2 the reflected beam isnot shown. For this arrangement the equivalent aperture of the mirror isa flat surface A (shown dotted) in front of it and inclined to the saidmirror, being nearest thereto at the top and furthest away at thebottom. The shortest ray path from flare to equivalent aperture is thatfrom the middle of the flare mouth to a point of reflection at themiddle of the mirror, all the other paths being longer. To avoid thedefect of inequality of phase distribution at the aperture the mirror isprovided, in accordance with this invention, with a series of partitionplates W which, in effect, subdivide the space immediately in front ofthe mirror into a series of short wave guide sections of varyinglengths. In the present case of horizontal polarization the partitionplates W, or correcting plates, as they may be termed, are horizontaland parallel to one another and to the electric fleld from the flare,being directly attached, edge on to the mirror proper M, which may be aflat solid conductor or made of mesh or gauze or wire in any convenientwell known way.

The plate edges further from the mirror are curved (see Figure 4) sothat each is of minimum length (i. e. the dimension in the direction ofenergy propagation) at the vertical centre line of the mirror. Similarlythe lengths of the separate plates are varied from top to bottom of themirror (see (Figure 5), being of minimum length at the horizontal centreline. Assuming the spacing of the plates to be chosen equal throughoutand such as to make the velocity between them twice the free spacevelocity (these two conditions are not necessary conditions but providethe easiest case to consider) the plates may be so arranged that theircurved edges lie in a substantially spherical surface oppositely curvedwith respect to and thus compensatory for, an imaginary curved surfacetangential to the equivalent aperture and at which reflected rays ofequal length from the flare mouth termlnate. Since the extent ofvelocity modification produced by any pair of plates is a function oftheir spacing, if the plates are not equally spaced or are not spaced toproduce twice the free space velocity, their shapes must be modifiedfrom that just described to produce the same results. In order toeconomize in metal however, the construction just described may be, andpreferably is, modified as shown by stepping each plate back by a wavelength at points Ll, L2, L3, L4 where its length (its dimension in thedirection of propagation) is equal to one wave length plus the length ofthe centre plate at its middle, 1. e., where the shortest ray path issituated. This, of course, will result in different point on theequivalent aperture having phases dilfering by an integral number ofwavelengths, but this is equivalent to equal phase distribution. InFigure 4 the unstepped construction is indicated by the broken linecontinuation of the central portion of the plate therein shown.

It may be noted that each ray passes twice through each wave guidesection provided by the plates. This, of course, involves a shorterlength of plate (half) than would otherwise be necessary.

In another embodiment illustrated in diagrammatic front elevationan-dplan in Figures 6 and 7 respectively, and in diagrammatical centralsectional elevation (on the line Z-Z of Figure'6) in Figure 8 th energysource P approximates to a line and is constituted by the wide shallowmouth of a flat, flared horn flaring out from a Wave guide WG and bentover to direct energy towards a rectangular mirror M which is, asbefore, positioned to reflect the beam back over the upper edge of thehorn. This mirror is also fitted with horizontal partition plates Wshaped and arranged in accordance with the principles already described.

Figure 9, which Willbe found self-explanatory in view of the descriptionalready given, shows an embodiment for a vertically polarized systemdiffering from the preceding embodiments, of course, in that the platesW extend vertically.

In Figure 9 the mirror M is oriented about a vertical axis so that thereflected beam clears the wave guide flare P.

.In the foregoing description it has been assumed that the requirementis to produce an equiphase surface at the equivalent aperture. This willusually be the case but is not always so: for some purposes it may berequiredto produce a phase warp to satisfy some particular polar diagramrequirement. Again, it has been assumed throughout that the energysource is either approximately a point source or an area source givingequiphase energy everywhere in that area. This may not always be thecase and the phase distribution across the source must be taken intoaccount when designing a mirror in accordance with this invention. Forexample, when employing this invention in conjunction With a horn inaccordance with the invention contained in the specification of Britishpatent application No. 8,544/46 part of a total required phasecorrection could be applied by the horn and part by the mirror.

An important advantage of the particular embodiments herein described isthat the mirrors proper are flat and the section shapes are not socritical as would be the case with, for example, parabolic mirrors;moreover, by appropriate design, the edge shapes can be made circular.These advantages are of great importance for mirrors for use oncentrimetre and millimetre wave lengths. However, the invention is notlimited exclusively to its application to flat mirrors though this isprobably the most important case of the invention practically.

In some cases, in order to avoid defects due to the bending of ray pathsby refraction it may be found advantageous to provide additional metalplates B normal to the direction of the electric field as shown in Fig.10, and serving to guide the wave elements. The additional metal platesB, which may be termed baflie plates to distinguish them from thevelocity modifying partition plates to which they are at right angles,will, if provided, result in a honeycomb-like or cellularstructure.

In constructions in accordance with this invention wherein partitionplates are provided the beam is, in efiect, divided into wave guidesecof the same transverse dimensions over its length but if desired thepartition plates may be :so mounted as to taper the wave guide sectionsover part or the whole of their lengths. If'this expedient be adopted itwill be appreciated that the velocity of propagation will be graduallymodified within the sections.

In general the plates (bailleand partition plates alike) employed incarrying out this invention, will be for mechanical reasons, solidconductive plates. If desired for reasons of lightness-or reduction ofwindage, however, the plates like-the mirror proper, may b perforated or"formed :of suitable mesh gauze or .wire structures and .the termsplate, mirror, reflector surface, as herein employed,.are intended in awide sense to include such structures.

In .the embodiment specifically described and illustrated theinter-plate spaces are air spaces, 1. e. air dielectric is employed inthe wave-guide sectionseifeotively formed. This, however, is-notessential and if desired the inter-plate spaces may be wholly or partlyfilled with suitablesolid dielectrics, due regard being of course, paidin design to the effect of such dielectric on propagation velocity. Suchuse of solid dielectric offers advantages in many cases from the pointof.view of mechanical strength and streamlining for reduced windage.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to beperformed, wedeclare thatwhat We claim is:

vl. A radio mirror arrangement comprising a radio reflector having areflecting surface, and-a plurality of spaced parallel metallic-surfacedpartition plates arranged in front of said surface and extending acrossit so as to divide the space in front of said surface in effect into aplurality of wave grid sections, said plates being supported edge-on tosaid reflecting surface, the distance of separation between each pair ofadjacent plates being greater than the separation affording cutoff atthe operating frequency, the depth of each plate from edge to edgevarying along the length of the plate.

2. The arrangement claimed in claim 1, wherein the said distances ofseparation of adjacent pairs of plates are all equal.

3. The arrangement claimed in claim 1, the plates being stepped back atpoints along their length where their depths, at such points, withoutthe step-back, would provide an integral number of wavelengths measuredat the phase velocity between plates for energy at the operatingfrequency plus a predetermined depth equal to the least depth of anyplate.

4. The arrangement claimed in claim 1, the radio reflector surface beingflat.

5. The arrangement as claimed inclaim 1, and comprising also baffleplates spaced from and parallel to each other and at right angles to thesaid partition plates.

6. A radio Wave reflector arrangement comprising a substantially flatreflecting surface, an antenna element arranged in cooperative radiowave translation relationship to said reflecting surface, said antennaelement and said reflecting surface normally acting on radio Waves toproduce a substantially spherical Wave front, and wave velocitymodifying means arranged between said antenna and said reflectingsurface and comprising a plurality of partition members and arrangededgewise'to said reflecting surface and parallelto the direction of theelectric fleld, the width of each member varying along the lengththereof and the width of said members at corresponding loci varying withrespect to each other to differentially modify the velocity ofpropagation of the radio waves translated over different paths betweensaid antenna element and said reflecting surface to produce apredetermined phase distribution in a given plane tangential to a givendirection of said spherical wavefront.

'7. A radio Wave reflector arrangement comprising a substantially flatreflecting surface, an antenna element arranged in cooperative radiowave translation relationship to said reflecting surface, said antennaelement and said reflecting surface normally acting on radio waves toproduce a substantially spherical wave front, and wave velocitymodifying means arranged between said antenna element and saidreflecting surface and comprising a plurality of partition membersarranged edgewise to said reflecting surface and parallel to thedirection of the electric field, the width of each member varying alongthe length thereof and the width of said members at corresponding locivarying with respect to each other to differentially modify the velocityof propagation of the radio waves translated over different pathsbetween said antenna element and said reflecting surface to produce apredetermined phase distribution in a given plane normal to the desireddirection of said cooperative radio wave translation.

8. A radio wave reflector arrangement comprising a substantially flatreflecting surface, an antenna element arranged to radiate radio wavesto said reflecting surface for propagation therefrom by reflection, saidantenna element and said reflecting surface normally acting on saidradio waves to produce a substantially spherical wave front, and wavevelocity modifying means arranged between said antenna element and saidreflecting surface and comprising a plurality of partition membersarranged edgewise to said reflecting surface and parallel to thedirection of the electric field, the width of each member varying alongthe length thereof and the width of said members at corresponding locivarying with respect to each other to differentially modify the velocityof propagation of said radio waves over different paths between saidantenna element and said reflecting surface to produce a predeterminedphase distribution in a given plane normal to the desired direction ofpropagation.

9. A radio wave reflector arrangement comprising a substantially flatreflecting surface, an antenna element arranged to radiate radio Wavesto said reflecting surface for propagation therefrom by reflection, saidantenna element and said reflecting surface normally acting on saidradio waves to produce a substantially spherical wave front, and wavevelocity modifying means arranged between aid antenna element and saidreflecting surface and comprising a plurality of partition membersarranged edgewise to said reflecting surface and parallel to thedirection of the electric field, the width of each member varying alongthe length thereof and the width of said members at corresponding locivarying with respect to each other to differentially modify the velocityof propagation of said radio waves over different paths between saidantenna element and said reflecting surface to produce a predeterminedphase distribution in a given plane normal to the desired direction ofpropagation, said reflecting surface being tilted with respect to theaxis of propagation of said antenna element to prevent energy beingreflected back thereto.

10. A radio wave reflector arrangement comprising a substantially flatreflecting surface, an antenna element arranged to radiate radio Wavesto said reflecting surface for propagation therefrom by reflection, saidantenna element and said reflecting surface normally acting on saidradio Waves to produce a substantially spherical wave front, and wavevelocity modifying means arranged between said antenna element and saidreflecting surface and comprising a plurality of partition membersarranged edgewise to said reflecting surface and parallel to thedirection of the electric field, the width of each member varying alongthe length thereof to present path distances between said antennaelement and said reflecting surface which in the absence of saidvariation would be equal to a whole number of wavelengths includingunity plus a predetermined length equal to the minimum width of thenarrowest of said members, the width of said members at correspondingloci varying with respect to each other to differentially modify thevelocity of propagation of said radio waves over different paths betweensaid antenna element and said reflecting surface to produce apredetermined phase distribution in a given plane normal to the desireddirection of propagation, and baffle plates arranged normally to saidpartition members and interposed therebetween, said reflecting surfacebeing tilted with respect to the propagational axis of said antennaelement to prevent energy being reflected back thereto.

11. A radio wave reflector arrangement comprising a reflector elementhaving a reflecting surface, an antenna element arranged to illuminateor receive radiation at an operating frequency from said reflectingsurface, and wave velocity modifying means comprising spaced metallicplates parallel to each other and to the electric field of the radiationand arranged between said antenna element and said reflecting surface,and also arranged between said reflecting surface and the space to orfrom which waves are reflected from said element or received by saidsurface to modify the velocity of propagation of the radio wavestranslated over different paths between said space and said antennaelement both before and after reflection on said surface, saidwave-velocity modifying mean comprising a plurality of parallelmetallic-surfaced plates spaced from and parallel to each other andedgeon to said reflecting surface, the space between any adjacent pairof plates exceeding the separation affording cut-oif at the operatingfrequency, the depth of said plates edge to edge varying along thelength thereof, whereby the said waves pass through said modifying meanstwice, to produce a predetermined phase distribution in a given plane.

NOEL MEYER RUST. MICHAEL CRAVEN GREGORY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Hansell July 8, 1947 OTHER REFERENCESNumber

